1. Field of the Invention
The present invention generally relates to mobile radio communication systems and, more particularly, is directed to a radiotelephone communication system in which a fundamental principle of the cellular phone system can be realized.
2. Description of the Prior Art
In a small zone type mobile telephone system, that is, a so-called cellular phone system, a transmission distance of radio waves is minimized and a frequency is repeatedly utilized from a plane standpoint (from an area standpoint) to thereby increase telephone subscriber capacity of the cellular phone system.
As, for example, shown in FIG. 1, the service area is divided to provide a plurality of divided areas A.sub.1 to A.sub.m and the divided areas A.sub.1 to A.sub.m are divided to provide a plurality of radio zones (base station areas) Z.sub.1 to Z.sub.n. Base stations B.sub.1 to Bn are provided in these radio zones Z.sub.1 to Z.sub.n, and a mobile station (radiotelephone) HS is provided in the radio zone Z.sub.9.
In that case, control channels of different frequencies are allocated to the base stations B.sub.1 to B.sub.n, but when base stations are located in different areas and in the zones close to each other, control channels of different frequencies are allocated to the base stations B.sub.1 to B.sub.n. The base stations B.sub.1 to B.sub.n are networked each other via a computer (not shown) which controls the base stations B1 to Bn, and communication channels of the base stations B.sub.1 to B.sub.n are allocated.
In the case of FIG. 1, the mobile station HS is located within the zone Z.sub.3 so that, when the mobile station HS transmits an outgoing call request signal, the base station B.sub.3 of the zone Z.sub.3 receives the transmitted wave from the mobile station HS at the largest electric field intensity and the base stations in other zones receive the transmitted wave from the mobile station HS at electric field intensity smaller than that of the base station B.sub.3.
Accordingly, the control computer determines on the basis of the difference of electric field intensities that the mobile station HS is located in the base station B.sub.3, and issues a command to the base station B.sub.3 to open a communication channel between it and the mobile station HS. As a consequence, the communication channel is opened between the base station B.sub.3 and the mobile station HS and the mobile station HS is connected through the base station B.sub.3 to the telephone line network (not shown).
In the cellular phone system, the base stations B.sub.1 to B.sub.n may be connected to the mobile station HS in their own zones, and the mobile station HS may be connected to the base station of one zone, thereby reducing the transmission distance of radio waves between the base stations B.sub.1 to B.sub.n and the mobile station HS as much as possible. Therefore, the communication channel of the same frequency is repeatedly utilized even in the same service area, thus increasing the telephone subscriber capacity of this system.
In recent years, a small pocketable mobile station (radiotelephone) is commercially available so far, and accordingly, it is frequently observed that the user utilizes the mobile station in various places, such as mountain, highest floor of skyscraper, aircraft and so on.
However, when the mobile station is utilized in the high place, or when the mobile station HS makes an outgoing call request on the mountain as, for example, shown in FIG. 2, a transmitted radio wave from the mobile station HS is received by the base stations B.sub.1 to B.sub.n at substantially the same electric field intensity or such transmitted radio wave is similarly received by other areas.
In this state, it is difficult to repeatedly utilize the frequency from an area standpoint and the fundamental idea of the cellular phone system or the principle itself is refused.